In the field of radiography, where conventional silver halide films are used to capture radiographic images, it is common to enclose an x-ray film, sensitized on both surfaces in a light tight cassette, between two intensifying screens of the type customarily described as prompt emission stimuable phosphor intensifying screens. Higher resolution images are produced by using an x-ray film sensitized on only one surface, in contact with a single intensifying screen.
As most radiographic procedures are carried out in normal room lighting, a primary requirement for an x-ray cassette is to shield the film from exposure by ambient light.
Since silver halide x-ray films are relatively insensitive to exposure by electromagnetic radiation in the wavelength range generally referred to as x-rays, intensifying screens are utilized to capture and convert x-rays into visible wavelength radiation, to which the film is relatively sensitive.
Prompt emission stimuable phosphor intensifying screens are manufactured by coating a layer of phosphor particles contained in a binder, such as latex, onto a smooth substrate. The substrate is chosen to be relatively flat, yet flexible, such as a polyester film with a thickness in the range of 0.004" to 0.015". The phosphors so chosen for such prompt emission stimuable phosphor intensifying screens are selected to absorb x-rays in the wavelength range normally used for medical x-ray procedures, and to emit visible wavelengths in the blue-green part of the spectrum, when said phosphors promptly return to their unexcited state, emitting said visible light substantially in all directions.
If a prompt emission stimuable phosphor intensifying screen is held in intimate contact with the sensitized surface of an x-ray film, then the light sensitive film only in close proximity to the point from which the emission occurred is exposed by said emission, thus sharply imaging each such point, whereas, if the surface of the intensifying screen is not in intimate contact with the film, the light emitted from each point within the intensifying screen will spread and expose a larger area on the film, overlapping the exposures from adjacent points within the intensifying screen, resulting in an unsharp image upon the film.
Thus, for cassettes in use as general radiography cassettes, an essential secondary requirement is to provide for intimate contact between an intensifying screen contained within the cassette and an x-ray film placed therein.
In a normal radiographic procedure, the patient, or portion thereof, is placed between an x-ray source, and an x-ray cassette containing two intensifying screens. When the patient is irradiated, the x-ray beam is selectively attenuated by differences in the density of the patient tissue and bone structure. When the imaging beam strikes the x-ray cassette, a portion of the beam is attenuated by the front panel. The remaining imaging beam enters the cassette, where a significant portion is absorbed by the first intensifying screen, which promptly emits visible light against the first sensitized surface of the film. The remaining x-ray beam then passes through the film, causing a small direct exposure to both sensitized surfaces. The majority of the yet remaining portion of the beam is absorbed by the second intensifying screen, which then emits visible light against the second sensitized surface of the film. The residual small portion of the imaging beam is further attenuated by a thin sheet of lead foil applied to the interior face of the back panel, leaving only a very small fraction of the original beam to escape through the back surface of the cassette.
To minimize irradiation of the patient, it is desirable to minimize the attenuation of the imaging beam by non-image forming portions of the x-ray cassette, thus, a third requirement for x-ray cassette design is the utilization, for the front panels of such cassettes, of materials which do not substantially attenuate an x-ray beam. Additionally, such front panels must provide for uniform attenuation, so as to not alter the pattern of attenuation created by the patient.
Materials which are lower in atomic weight are more transparent to x-rays, thus beryllium would be an excellent material for x-ray cassettes, were it not for its great cost, and toxicity. Thermoplastic materials which are primarily compounds of carbon, hydrogen, and oxygen are also suitable, although they generally lack structural strength and modulus of elasticity required to produce good film screen contact in larger size cassettes. Carbon fiber reinforced thermoset resins are commonly used, in spite of their great cost. Aluminum is generally suitable, in the wavelengths used for general radiography, however it is unsuitable at the longer wavelengths used in specialty procedures, such as mammography.
Commonly available x-ray cassettes comprise a front cover hingeably attached along a first edge to a back cover, with latch means provided along a second edge opposite said first edge, to maintain closure of said cassette, during an x-ray procedure, and to allow opening of said cassette for loading an unexposed x-ray film, and for removing an exposed x-ray film. Light lock means are provided around the perimeter of said covers to render said cassette light tight when said cassette is closed. Intensifying screens are each adhered to resilient foam pads, which are subsequently adhered to the interior surfaces of said covers. The thickness of the foam pads, and the stiffness of the foam material is chosen such that the form pads are partially compressed within the closed cassette, thus urging the intensifying screens into contact with an x-ray film placed therebetween.
Higher levels of contact pressure between screens and film produce more intimate contact, and help squeeze out air that may have been trapped between the film and screen surfaces when the cassette was closed, said entrapped air preventing intimate film/screen contact.
Higher internal pressures also result in greater cassette deformation, resulting in non-uniform internal pressures, and potential separation of perimeter light lock elements.
Currently available cassettes represent a series of compromises within which the various cassette designers have variously balanced the desire to have intimate film/screen contact against the practical aspects of the structural requirements dictated by high contact pressures, the attenuation levels of structurally superior materials, and the cost of cassette materials and manufacturing processes.
An example of one such commonly available general purpose radiographic cassette, manufactured by the Eastman Kodak Company, of Rochester, N.Y., and sold under the name "Kodak X.sub.-- OMATIC Cassette", comprises two aluminum panels 0.063" thick, each being insert molded into a thermoplastic border material which hingeably connects the two panels, along one edge of each, and forms a series of cooperating perimeter ribs and grooves around three sides of each panel, for the purpose of excluding light from the closed cassette. An "L" shaped stainless steel structural member is attached to each of the two remaining edges with up to 24 rivets in the larger size cassettes. Two aluminum extrusions are also attached at the said remaining edges, to provide light lock structure along the fourth side of the closed cassette. One of said extrusion is attached adhesively, the other in conjunction with the process of attaching one of the "L" shapes, and utilizing the same rivets as used to attach the "L" shape. Latching of the cassette in the closed position is accomplished by engaging a latch piece, spot welded to one of the "L" shapes, with a hook, suspended from a pin, said pin supported by a knuckle plate, said knuckle plate adhesively bonded to the second "L" shape. A lever, also suspended from said pin is rotated to deflect said hook from its engaged position, to enable opening the cassette. Two springs urge the hook, and the lever, toward their first positions whereby the cassette is latched. A recess in one of the aluminum panels is formed by a stamping process, to provide clearance under the lever, to allow manual operation of the lever, when the cassette user desires to open the cassette.
To complete the cassette, a layer of lead foil is applied to the interior face of the back panel, and foam pads are then adhesively bonded to the interior of the first cover, and atop the lead foil, and the intensifying screens are adhesively bonded to the opposite sides of the foam pads, such that the phosphor coated surfaces of said screens are urged against one another, or against an x-ray film placed therebetween, when the cassette is closed and latched.
Prior to assembly, the aluminum panels are formed into a curved shape, to enable them to act as springs, applying pressure to the foam pads, to partially compress them, substantially uniformly, when the cassette is closed, for the purpose of applying pressure to the intensifying screens to urge them into intimate contact with the x-ray film placed within the cassette. Pressure in the range of 0.10 to 0.15 psi is attained in the currently marketed cassette, and this level of pressure demonstrates intimate contact over the entire area of the x-ray film, when tested in accordance with methods prescribed by the American College of Radiography.
The front cover of a cassette of the above described construction will attenuate 26% of an imaging beam generated at the x-ray wavelengths created by a 100 kv excitation voltage applied to an x-ray generator, with 3 mm aluminum filtration at the generator, with no scatter control, and with no other absorber in the beam. At slightly longer wavelengths generated at 80 kv excitation, the attenuation is 30%, and at yet longer wavelengths generated at 60 kv excitation, the attenuation reaches 36%.
Some cassettes are manufactured utilizing carbon fiber reinforced epoxy panels of equivalent structural performance, for the front cover only. Such carbon fiber panels will typically attenuate 1%, 2% and 2.5% of the above imaging beams, a significant decrease in attenuation, albeit, at a panel cost of ten to twenty times the cost of the vinyl clad aluminum panel it replaces.
The above-described cassette, featuring two vinyl clad aluminum panels, completely assembled, and including film and intensifying screens, in a 35.times.43 cm. size, as is commonly used for procedures such as chest x-ray, comprises 49 discrete components, including fasteners and adhesive applications, and weighs 5.70 pounds. Such a cassette has proven to be robust in usage, remaining serviceable for 10 years or more, experiencing many thousands of reloadings, an equivalent number of insertions and removals from various x-ray apparatus, occasional accidental drops, and in some cases, innumerable instances where the cassette is placed directly under a patient, and bears the weighs of the patient, as when the cassette is used for portable radiography at bedside.
While the above described cassette is widely utilized, it would be desirable to provide x-ray cassettes utilizing lower cost materials and/or processes; having equally high internal contact pressures, to ensure excellent image quality; while minimizing the imaging beam attenuation of the front panel of the cassette, to reduce patient irradiation levels. It would be further desirable to provide a cassette which is structurally equivalent to the current cassette; and which is lighter in weight, for portability. Furthermore, it would be desirable to reduce the number of discrete parts comprising an x-ray cassette assembly, to further reduce weight, and to further reduce parts manufacturing and assembly costs.